Water shortage has already become a prominent global challenge since we entered the 21st century, an age with accelerating urbanization, overpopulation and increasingly severe environmental pollution. It has become a great concern in China as well. The aggravating crisis of fresh water shortage calls for advanced technologies for treating agricultural wastewater, industrial wastewater and urban domestic sewage, particularly those that can realize deep purification of wastewater by removing most of inorganic salts, organic pollutants, total phosphorus and total nitrogen contained therein so that the high quality of reclaimed water required for multi-purpose utilization can be obtained. Development of these advanced technologies is consistent with sustainable development strategies and beneficial to communities and environmental improvement.
Biological methods are widely adopted in China to treat the agricultural wastewater, industrial wastewater and urban domestic sewage. However, with increasingly stringent standards stipulated by the Chinese government in respect of effluent discharge and reuse, it is necessary for biotreated effluent to undergo an advanced treatment process so that chromaticity, organic pollutants, total phosphorus, total nitrogen and the total salt content contained therein can be further reduced. Needless to say, efficient reuse of reclaimed water will reduce the total quantity of discharged wastewater and is consequently beneficial to the protection and conservation of water resources.
Desalting processes currently available for advanced wastewater treatment mainly include two conventional categories: the membrane process and the thermal process; the membrane desalting process mainly involves techniques such as reverse osmosis and electrodialysis; it presents excellent desalting effect, but it requires high investment and operating cost and bears a limitation that the membrane used in the process is susceptible to contamination; the thermal desalting process mainly involves techniques such as multistage flash evaporation, low-temperature multi-effect distillation, vapor compression distillation, but like the membrane desalting process, it requires high investment and operating cost as well. Another desalting process newly emerged in the field of advanced wastewater treatment is electrosorption, which realizes adsorption of ions by creating double-layer capacitance on the surface of an electrode supplied with a direct voltage less than 2 v. It is a highly effective and economically favorable process characterized by low cost and high efficiency. However, the organic pollutants in the wastewater may cause great damage to the electrosorptive electrode and therefore shorten its service life; besides, organic ions will compete with inorganic ions for adsorptive sites, which consequently lowers down the desalting efficiency of electrosorption. Therefore, when used alone for advanced wastewater treatment, electrosorption cannot guarantee the high-quality reclaimed water being obtained in an efficient and economical way.
A Chinese patent (publication number: CN 102452751A; publication date: May 16, 2012) entitled “A Method for Advanced Desalting and Reuse of Industrial Wastewater” disclosed a method for advanced desalting and reuse of industrial wastewater which is characterized in that the untreated industrial wastewater is sequentially treated in a hardness-reducing unit, a highly effective filtering unit and an electrosorptive desalting unit so that the effluent obtained therefrom can reach the reuse standards. The steps included in the process are (1). hardness reduction, precipitation and neutralization, (2). highly effective filtration; (3). electrosorptive desalting. This method requires no electrodialysis or reverse osmosis membrane devices, and it requires no ion exchange resin either. Besides, the whole system can work stably and without scale formation when the electrical conductivity of the water is higher than 2500 μS/cm. When this method is adopted to treat the biotreated effluent of industrial wastewater with electrical conductivity lower than 4500 μS/cm, the final effluent obtained therefrom can be used as water supply of an industrial recycled water system. It can be used for advanced treatment of chemical wastewater, dyeing wastewater, textile wastewater and refinery wastewater, and presents a variety of advantages such as wide applicability for various types of wastewater, stable operational performance and high desalting efficiency. However, this method is not suited to treat urban domestic sewage, particularly those types of wastewater that contains high concentration of organic pollutants, as the organic pollutants in the wastewater will cause great damage to the electrosorptive electrode and therefore shorten its service life; besides, organic ions contained therein will compete with inorganic ions for adsorptive sites, which consequently lowers down the desalting efficiency of electrosorption. Therefore, when used alone for advanced wastewater treatment, the electrosorption process disclosed in embodiments of the invention cannot guarantee the high-quality reclaimed water being obtained in an efficient and economical way. A new method need to be found for advanced treatment of wastewater with high concentration of organic pollutants, particularly a method simultaneously presenting all advantages of electrosorption and ensuring long service life of the electrode so that the high-quality reclaimed water can be obtained in an efficient and economical way.